Work is continuing on two projects related to the J chain gene. In our study of human J chain gene regulation, we are continuing the analysis of the promoter of this gene by assessing the effects of small deletions..on promoter function in gene constructs transfected into B lymphocytes. In our collaborative project concerning J chain function, a mouse J chain gene construct containing an insertion that inactivates the gene has been successfully inserted into mouse embryonic stem (ES) cells by homologous recombination, inactivating one of the two copies of the J chain gene. One of these cell lines is currently being injected into mouse blastocysts in an effort to develop a J chain-less mouse strain; and additional independent recombinant ES cells are being sought. In a nev project relating to the regulation of human immunoglobulin kappa gene expression, we have identified a functional enhancer downstream of the human kappa constant region gene and have narrowed down the functional region to about 0.2 kb by deletion constructs. We have detected sequence specific binding of lymphocyte nuclear protein(s) to the same region by gel retardation analysis. Current work on this project is aimed at identifying the exact sequences necessary for enhancer function. Two collaborative studies have been initiated relating to the regulation of immunoglobulin isotype switching by the lymphokine IL-4. In a system of IL-4 induced IgE production in human EBV-transformed lymphocytes, we have shown that, in contrast to an earlier report, the IgE expression is accompanied by a typical isotype switch rearrangement that deletes mu genes and rearranges at least one epsilon gene; work is continuing to identify early steps in the rearrangement. In a system of IL-4 induced IgGl production in LPS-stimulated murine splenic B lymphocytes, we have developed a sensitive assay for switching to the gamma gene and have preliminary evidence that surface expression of IgG1 precedes isotype switch rearrangement in most cells.